Compare dropdown fails when selecting branch with no common merge-base (#37470) (#37472)

## Summary

- handle compare requests where base and head refs have no common merge
base without returning 500
- keep the compare branch selectors usable and show a clear warning
message
- add regression coverage for unrelated-history compare selection and
merge-base error detection

Fixes #37469 



Manuel Backport of: https://github.com/go-gitea/gitea/pull/37470

---------

Co-authored-by: Codex <codex@openai.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/repo/compare.go

@@ -413,6 +413,10 @@ func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
 
 	compareInfo, err := git_service.GetCompareInfo(ctx, baseRepo, headRepo, headGitRepo, baseRef, headRef, compareReq.DirectComparison(), fileOnly)
 	if err != nil {
+		if errors.Is(err, util.ErrNotExist) {
+			ctx.Data["IsNoMergeBase"] = true
+			return compareInfo
+		}
 		ctx.ServerError("GetCompareInfo", err)
 		return nil
 	}
@@ -603,9 +607,18 @@ func CompareDiff(ctx *context.Context) {
 	ctx.Data["PullRequestWorkInProgressPrefixes"] = setting.Repository.PullRequest.WorkInProgressPrefixes
 	ctx.Data["CompareInfo"] = ci
 
-	nothingToCompare := PrepareCompareDiff(ctx, ci, gitdiff.GetWhitespaceFlag(ctx.Data["WhitespaceBehavior"].(string)))
-	if ctx.Written() {
-		return
+	var nothingToCompare bool
+	noMergeBase := ctx.Data["IsNoMergeBase"] == true
+	if noMergeBase {
+		ctx.Flash.Error(ctx.Tr("repo.pulls.no_common_history"), true)
+		ctx.Data["PageIsComparePull"] = false
+		ctx.Data["CommitCount"] = 0
+		nothingToCompare = true
+	} else {
+		nothingToCompare = PrepareCompareDiff(ctx, ci, gitdiff.GetWhitespaceFlag(ctx.Data["WhitespaceBehavior"].(string)))
+		if ctx.Written() {
+			return
+		}
 	}
 
 	baseTags, err := repo_model.GetTagNamesByRepoID(ctx, ctx.Repo.Repository.ID)
@@ -621,16 +634,13 @@ func CompareDiff(ctx *context.Context) {
 		return
 	}
 
-	headBranches, err := git_model.FindBranchNames(ctx, git_model.FindBranchOptions{
-		RepoID:          ci.HeadRepo.ID,
-		ListOptions:     db.ListOptionsAll,
-		IsDeletedBranch: optional.Some(false),
-	})
+	headBranches, headTags, err := getBranchesAndTagsForRepo(ctx, ci.HeadRepo)
 	if err != nil {
-		ctx.ServerError("GetBranches", err)
+		ctx.ServerError("GetBranchesAndTagsForRepo", err)
 		return
 	}
 	ctx.Data["HeadBranches"] = headBranches
+	ctx.Data["HeadTags"] = headTags
 
 	// For compare repo branches
 	PrepareBranchList(ctx)
@@ -638,12 +648,10 @@ func CompareDiff(ctx *context.Context) {
 		return
 	}
 
-	headTags, err := repo_model.GetTagNamesByRepoID(ctx, ci.HeadRepo.ID)
-	if err != nil {
-		ctx.ServerError("GetTagNamesByRepoID", err)
+	if noMergeBase {
+		ctx.HTML(http.StatusOK, tplCompare)
 		return
 	}
-	ctx.Data["HeadTags"] = headTags
 
 	if ctx.Data["PageIsComparePull"] == true {
 		pr, err := issues_model.GetUnmergedPullRequest(ctx, ci.HeadRepo.ID, ctx.Repo.Repository.ID, ci.HeadRef.ShortName(), ci.BaseRef.ShortName(), issues_model.PullRequestFlowGithub)

routers/web/repo/pull.go

@@ -1366,6 +1366,10 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 	if ctx.Written() {
 		return
 	}
+	if ctx.Data["IsNoMergeBase"] == true {
+		ctx.JSONError(ctx.Tr("repo.pulls.no_common_history"))
+		return
+	}
 
 	validateRet := ValidateRepoMetasForNewIssue(ctx, *form, true)
 	if ctx.Written() {